1054, Schism ofalso called East–West Schismevent that precipitated the final separation between the Eastern Christian churches (led by the patriarch of Constantinople, Michael Cerularius) and the Western Church (led by Pope Leo IX). The mutual excommunications by the Pope and the Patriarch that year became a watershed in church history. The excommunications were not lifted until 1965, when Pope Paul VI and Patriarch Athenagoras, following their historic meeting in Jerusalem in 1964, presided over simultaneous ceremonies that revoked the excommunication decrees.

The relation of the Byzantine Church to the Roman may be described as one of growing estrangement from the 5th to the 11th century. In the early church three bishops stood forth prominently, principally from the political eminence of the cities in which they ruled—the bishops of Rome, Alexandria, and Antioch. The transfer of the seat of empire from Rome to Constantinople and the later eclipse of Alexandria and Antioch as battlegrounds of Islām Islam and Christianity promoted the importance of Constantinople. Concurrently, the theological calmness of the West, in contrast to the often violent theological disputes that troubled the Eastern patriarchates, strengthened the position of the Roman popes, who made increasing claims to preeminence. But this preeminence, or rather the Roman idea of what was involved in it, was never acknowledged in the East. To press it upon the Eastern patriarchs was to prepare the way for separation; to insist upon it in times of irritation was to cause a schism.

The theological genius of the East was different from that of the West. The Eastern theology had its roots in Greek philosophy, whereas a great deal of Western theology was based on Roman law. This gave rise to misunderstandings and at last led to two widely separate ways of regarding and defining one important doctrine—the procession of the Holy Spirit from the Father or from the Father and the Son—with the Roman churches, without consulting the East, incorporating the Son into their creed. The Eastern churches also resented the Roman enforcement of clerical celibacy, the limitation of the right of confirmation to the bishop, and the use of unleavened bread in the Eucharist.

Political jealousies and interests intensified the disputes; and at last, after many premonitory symptoms, the final break came in 1054, when Pope Leo IX struck at Michael Cerularius and his followers with an excommunication and when the Patriarch retaliated with a similar excommunication. There had been mutual excommunications before, but they had not resulted in permanent schisms. At the time there seemed possibilities of reconciliation, but the rift grew wider; in particular, the Greeks were bitterly antagonized by such events as the Latin capture of Constantinople in 1204. Western pleas for reunion (on Western terms), such as those at the Council of Lyon (1274) or the Council of Ferrara-Florence (1439), were rejected by the Byzantines. The schism has never been healed.